The invention relates to a method and arrangement for changing parallel clock signals in the propagation assurance of digital data transmission, particularly for realising the propagation assurance of radio links. The invention is suited to other data transmission connections as well, for instance to connections using optical transmission paths.
The quality requirements for a digital radio link are generally known; said requirements are set for example by the ITU, International Telecommunication Union. The quality requirements refer to the reliability and interference-free quality of the transmission. The most important features are usability, error ratio and phase noise. Among the factors that affect the fulfilment of said criteria are hardware malfunctions, weather and changes in the signal path. In order to fulfil the requirements, it is necessary to provide an equipment and propagation assurance for the radio link, which means the use of alternative equipment and transmission paths. By means of equipment assurance, there is obtained a more reliable usability, and by propagation assurance, there is obtained both a lower error ratio and a lower phase noise.
FIG. 1 is a block diagram illustrating one target of propagation assurance. A public switched telephone network (PSTN) 11 is connected by wires to a mobile switching centre (MSC) 12. The security of the radio link between the switching centre 12 and the base station controller (BSC) 13 is extremely important, wherefore it is generally assured. The controller 13 is further connected, by radio connections which can also be backed up, to base telecommunication stations (BTS) 14, 16, 18 and to their antennas 15, 17, 19.
The propagation assurance of radio links is realised by means of one or several parallel radio connections. Now in parallel with the major radio connection, there is constructed one or several other backup transmission paths that carry the same information. The transmission paths are preferably different in order to prevent possible interference caused by the terrain and/or weather changes from affecting both paths at the same time. Among the transmission paths, there is selected the one that has, in the prevailing conditions, a better signal at the station receiving the radio link. The applied criterion for the selection is generally the signal strength, but also the correctness of the parity of the received information. The changing of the transmission path is carried out by means of a specific changeover device, in a way that is as error-free as possible, by compensating both the dynamic and static phase differences caused by the propagation of the signals in different transmission paths. The block dealing with the clock signal is the most critical part in the changeover device.
Among the drawbacks with known analog arrangements for changing the clock signal are the required separate components; in order to be able to use them, there is needed space on the circuit board and they consume a remarkable amount of power.
Another drawback with known analog arrangements is their cumbersome tuning as a final step in the production.